In the semiconductor industry, there is a continuing effort to increase device density by scaling device size. The workhorse patterning technology over the past several decades has been optical lithography due to its high throughput and mature infrastructure. Conventional scaling of optical lithography is growing increasingly difficult as feature sizes continue to drop further below the available exposure wavelengths ushering in an era of “subwavelength” lithography. This situation is not likely to change in the future as commercially required feature sizes are shrinking much faster that the wavelengths of new exposure tools.
Sub-wavelength lithography has been enabled by the semiconductor industry by the introduction of a variety of optical resolution enhancement techniques (RETs) including optical proximity correction (OPC), off-axis illumination (OAI) and phase shift masks (PSMs). Phase shift mask methods offer the greatest resolution enhancement potential. Although increasing resolution, these RET methods can substantially increase mask costs which are difficult to amortize over moderate to low volume wafer production runs.
It is the purpose of this invention to offer a novel method of producing dense contact features. This method will allow the cost amortization of expensive RET photomasks over moderate to low production volumes. This method will also enhance resolution and increase the process latitude of fabricating dense contact patterns that are amoung the most difficult levels to pattern in the semiconductor industry.
In order to form small-dimensioned features, a variety of phase-shifting techniques have been proposed. In some of these methods, features are defined by forming open regions in an opaque layer on a mask or reticle (referred to generally as “mask” herein). The open regions transmit substantially all radiation incident thereon. Near or surrounding these open regions are phase-shifters that also transmit some or all of the radiation incident thereon, but which shift the phase of the radiation approximately 180° relative to the openings forming the features. In this way, the radiation from the phase shifter destructively interferes with the radiation from the field regions, providing enhanced contrast at the feature's edge.
In this process, a mask, or “reticle,” includes a semiconductor circuit layout pattern typically formed of opaque chrome, on a transparent glass (typically SiO2) substrate. A stepper, which includes a light source and optics/lenses, projects light coming through the reticle to image the circuit pattern, typically with a 4× to 5× reduction factor, on a photo-resist film formed on a silicon wafer. The term chrome refers to an opaque masking material that is typically but not always comprised of chrome. The transmission of the opaque material may also vary such as in the case of an attenuating phase shift mask. It is further noted that the relative phase of the transmitted light may vary, as in the case of a phase shift mask.
FIG. 1 is an example of a conventional optical projection lithography apparatus. As illustrated in FIG. 1, the optical projection lithography apparatus includes a light source 20, a photomask 22, and reduction optics 24. A wafer 26, having a layer of photo-resist 28 thereon, is placed within the optical projection lithography apparatus, and the light-source 20 generates a beam of light 21 that is incident upon the photomask 22. The reduction optics 24 reduces the light beam to cause a pattern 30 that exposes the photo-resist layer 28, creating the pattern 30 of reacted material in the resist layer 28. In this manner, a pattern 32, provided on the mask 22, is transferred to the photo-resist layer 28 on the wafer 26.
The photo-resist pattern 30 is then transferred to the underlying wafer 26 through standard etching processes using standard semiconductor fabrication techniques. Both positive and negative tone resists can be used to produce either positive or negative images of the mask pattern on the wafer.
FIG. 2 is a further example of a conventional photolithography methodology. As illustrated in FIG. 2, a mask 100 comprises a first region 101, which may be referred to as the 0° phase and a second region 102 which may be referred to as the 180° phase. As shown, the second region 102 is adjacent to the first region 101 along interface 105. Intensity curve 110 shows the intensity of radiation at the image plane, I, as a fraction of the intensity incident on the mask 100, I0.
As shown, the intensity 111 underneath region 101 away from the interface 105 is nearly equal to the intensity incident on region 101. Similarly, the intensity 112 underneath section 102 away from interface 105 is nearly equal to the intensity incident on region 102. However, underneath the interface 105 there is a sharp drop 115 in the intensity at the image plane due to the destructive interference between the radiation transmitted through regions 101 and 102.
The exposure conditions can be adjusted such that the portion of the photosensitive layer underneath interface 105 is substantially unexposed, while portions of the photosensitive layer under regions 101 and 102 away from interface 105 are substantially exposed. In the case of a positive photoresist layer, after exposure and development, a thin line of photoresist will remain in the region underneath interface 105, while the remainder of the photoresist layer will be removed.
In the case of a negative photoresist layer, after exposure and development, the unexposed region underneath interface 105 will be removed while photoresist under the remainder of the photosensitive layer will be hardened, and will remain after development. Thus, this conventional methodology method may be used to form a narrow line in a positive photoresist layer or a narrow opening in a negative photoresist layer.
In one conventional method, as described in U.S. Pat. No. 5,635,316 and U.S. Pat. No. 5,766,829, a photosensitive layer is exposed to a first reticle having a pattern of parallel lines defined by alternating phase regions to form a first set of parallel features. Then, the photosensitive layer is exposed to a second reticle having a pattern of parallel lines defined by alternating phase regions to form a first set of parallel features which are arranged substantially orthogonal to those features formed by the first reticle.
As a result of these two exposures, a small dimension latent image is formed at every intersection of the first and second sets of features. Finally, a third exposure is performed using a standard trim or contact mask to expose those latent images where no contacts are desired. This method can also form contact hole or pillar features depending on the resist tone used (positive for pillars, negative for contact holes).
A schematic example of simulated results of a single exposure conventional photolithographic process is further illustrated in FIGS. 3 and 4. In this simulation, a dense array of contact holes features is the desired result. FIG. 3 illustrates an example of a mask, (chromeless phase shift mask) having 180° phase region(s) 210 and a plurality of 0° phase regions 200, that can be used to exposed a substrate to realize a dense array of contact hole features. This mask, in this example, has a constant feature pitch of 250 nm.
The imaging performance of the mask of FIG. 3 was simulated for a typical conventional lithography process with a 248 nm stepper wavelength, an NA (numerical aperture) of 0.6, and annular illumination with partial coherence σI=0.6,σO=0.8, wherein a dense contact grid is subjected to a single exposure. The resulting image for the simulation region 220 of FIG. 3 is defined by the schematic illustration of FIG. 4. As noted below, the Figures used to in this application are not necessarily to scale, and thus, as set forth above, it is further noted that the features illustrated in FIG. 3 should be set apart by a constant feature pitch.
As illustrated in FIG. 4, the resulting image for the simulation region 220 is a plurality of areas 240 of high intensity peaking at high intensity sections 241 and a plurality of areas 200 of low intensity bottoming out at low intensity sections 201. In this simulation, the peak intensity was 0.58 I0 and the minimum intensity was 0.37 I0, wherein I0 is the original intensity of the light incident upon the mask. Thus, the contrast of this exemplary conventional photolithographic process is 0.21 (0.58−0.37) that is relatively low for a manufacturable process.
It is further noted that in any optical lithography technique, the resulting optical image intensity is a function of the proximity of features. Contrast is lost as feature pitch values decrease. As a result, the resulting size of features located in densely populated regions can be different than the size for those features that are isolated from the densely populated features. This is known as the “optical proximity” effect.
With respect to optical proximity effect, the critical dimension of features depends on feature density. Moreover, optical proximity effects can become more severe in sub-wavelength lithography. The optical proximity effects can result in dense lines 261 and isolated line 262 on wafer 260 being printed with different sizes, even if the same size on the mask, as illustrated in FIG. 10, or dense contacts 263 and isolated contact 264 on wafer 260 being printed with different sizes, even if the same size on the mask, as illustrated in FIG. 11. Since the performance and yield of the circuit depends on the size and size tolerance of the gates and contacts, this is an undesirable result.
Spatial frequency effects are caused by the “low-pass filter” behavior of a projection lithography lens wherein high spatial frequencies do not pass through the lens. This results in corner rounding and line end shortening. An example of this effect is illustrated in FIG. 12. As illustrated in FIG. 12, a desired image is represented by mask 2200, but the actual image pattern 265 on the wafer is shortened and rounded.
To compensate for optical proximity and spatial frequency effects, additional features have been conventionally introduced on the mask that can involve both printable as well as sub-resolution elements. In these methods, extra features such as serifs, mousebites, hammerheads, and scattering bars are added to the mask features in order to correct for optical proximity effects and other spatial frequency effects. These conventional methods involve sophisticated algorithms with very large data size, as different corrections are required for each separation distance between the features. For this reason, conventional feature size correction (“OPC” or optical proximity correction) is a costly and time-consuming process. Such methods also add substantial mask fabrication complexity. This can lower mask yield and increase mask cost.
Notwithstanding, the conventional methods described above present various drawbacks.
For example, in some of the conventional methods described above, which use a crossed double exposure of two phase shift grating masks to make contact arrays, the process requires the moving in and out of two different masks or reticles of the optical path or one mask requiring rotation between exposures to produce a two-dimensional pattern prior to the trimming process. This requirement of two different masks or one rotating mask introduces alignment problems into the photolithographic process that must be overcome.
Furthermore, the use the two different masks or one rotating mask to produce a two-dimensional pattern prior to trimming introduces a significant time delay between exposures so as to allow the precise alignment the masks or rotated mask prior to exposure.
In the conventional method described above, which uses a phase shift mask requiring only one exposure to make the dense contact array, the conventional process results in very low contrast for the dense contact features.
Lastly, the conventional methodologies, as described above, often utilize annular or other types of off-axis illumination that provides insufficient contrast for very dense contact features.
Therefore, it is desirable to provide a photolithographic process that can produce dense two-dimensional features without introducing alignment problems or significant time delays in the fabrication process. Furthermore, it is desirable to provide a photolithographic process that provides enhanced contrast properties.
Moreover, it is desirable to develop an imaging method that can produce dense two-dimensional features without introducing alignment problems or significant time delays in the fabrication process while mitigating optical proximity and spatial frequency effects without adding complex optical proximity correction features to the mask, while preserving the resolution enhancement aspects required by sub-wavelength lithography.